A need has arisen for low-cost, tunable optical filters. For example, one proposed architecture for the future telephone network based on optical fiber uses wavelength-division multiplexing (WDM). In WDM, the data of different communication channels (e.g., multiple voice channels, video channels, high-speed data channels) modulate optical carriers of different wavelength, and all the optical carriers are impressed upon a single optical fiber. The multiplexed optical signals are all distributed to many customer sites, each with its own receiver. Each receiver must be able to pick out one of the multiplexed signals. In a direct detection scheme, an optical filter passes only the selected optical carrier, and an optical detector detects the time-varying (data modulated) intensity of the filtered optical carrier. Preferably, this filter should be tunable so as to easily select different data channels. Channel spacings of as little as 1 nm are being proposed in the infra-red band of 1.3 to 1.5 .mu.m.
Diffraction gratings provide the required resolution but are too expensive and fragile for customer-premise use. It is desired that the tunability be purely electrical and include no moving mechanical parts. Acousto-optic filters have been proposed. They offer superior resolution, tuning range, and ruggedness. However, their cost remain moderately high, and they require significant amounts of expensive RF electrical power.
A liquid-crystal light modulator has been disclosed by Saunders in U.S. Pat. No. 4,779,959 and by Saunders et al. in "Novel optical cell design for liquid crystal devices providing sub-millisecond switching," Optical and Quantum Electronics, volume 18, 1986, pages 426-430. A modulator blocks or passes the input light. A filter performs a more complicated function by frequency selecting from a broad input spectrum, passing some components while blocking others. Saunders defines a 10 .mu.m optical cavity between two partially reflecting metallic mirrors and fills the cavity with a nematic liquid crystal. The mirrors also act as electrodes for the standard liquid-crystal display configuration in which an applied bias rearranges the liquid-crystal orientation. However, in the etalon configuration of Saunders, the applied bias in changing the effective refractive index of the liquid crystal also changes the resonance condition for the cavity. Both references include a graph showing a bias-dependent optical filtering. Saunders relies upon this effect to intensity modulate a beam of well defined frequency between two intensity values.
The liquid-crystal modulator of Saunders could be modified to be used as a tunable filter for a wide bandwidth signal. However, it would operate poorly. For a simple Fabry-Perot etalon, the transmission at a given wavelength .lambda. for radiation incident to the normal of the surface is given by ##EQU1## where .delta.=.phi.+k.sub.0 dn. Here .tau. and .rho. are the transmittance and the reflectance and, .phi. is the phase shift experienced upon reflection, d is the thickness of the uniaxial material, n is the refractive index along the director axis, and k.sub.0 is the magnitude of the wave-vector outside the layer. Equation (1) shows that the width of the transmission peak .DELTA..lambda. depends essentially on the reflectivity of the surfaces, while the overall transmission is dictated by the absorption losses.
Saunders uses silver mirrors having reflectivities in the range of 85-90%. His illustrated transmissions peak at about 50%, an acceptable value for some applications, but the peaks are relatively wide. The widths present little problem when the structure is used as a modulator for a well defined wavelength. However, Saunder's peaks are separated by only a few times the peak widths. Therefore, his structure would be effective at filtering only a very few channels. For a practically useful device such as those useful in multichannel systems, the reflectivity of the mirrors must be kept above 95%. The peak width of Saunders could be reduced by increasing the thickness of the silver mirrors, thus increasing the mirror reflectivity to above 90%. However, the increased thickness would increase the mirror absorption losses to the point where the peak transmission is unacceptably lowered.